This invention relates to controlling the viscosity of printing ink and is particularly useful in connection with flexographic and gravure printing presses.
In printing presses adapted to use the flexographic process inking systems are used having relatively few rollers for transferring the ink from the ink fountain roller to the plate cylinder which transfers the ink to a running web or the like.
Printing presses designed to utilize the flexographic process are characterized by the use of a resilient relief plate, relatively fluid inks, and inking systems which comprise relatively few rollers. In such systems the ink is generally metered by means of flooded nip. It has been found that, in such metering systems, the only fliud property which affects the thickness of the metered ink film is viscosity. In other words the more viscous the ink is, the thicker the ink film. Of course, when thicker ink films are applied less web length can be printed with a given volume of ink, assuming that speed, roller hardness and roller pressure are all maintained constant, and consequently there is a loss of ink mileage. The importance of viscosity has a very practical significance since flexographic inks contain relatively large amounts of volatile solvents. During the course of a press run, these solvents can and do evaporate rapidly causing an increase in ink viscosity and a corresponding decrease in ink mileage. Thus, one of the problems faced by flexographic press operators, in maintaining uniform print density and ink mileage, is to periodically monitor ink viscosity and to add solvent as necessary to maintain or control viscosity at a constant value.
Historically, controlling ink viscosity was done by the pressman using a Zahn Cup (i.e., Zahn Viscosimeter manufactured by Instrument Department, General Electric Company, West Lynn, Mass.) or similar manual viscometer. A Zahn Cup is a bullet-shaped 44 cc cup with a precision drilled orifice in its bottom. Viscosity is measured by dipping the cup into the liquid being tested, withdrawing it, and then measuring the time (usually in seconds) required for the full volume (44 cc) of liquid to flow out the orifice. Viscosity thus measured is expressed as the time in seconds for the fluid to drain out and the number of the Zahn Cup. Although typically, a flexographic ink may have a viscosity of 20 seconds, Zahn Cup #2 (about 30 centipoise or the consistency of milk) the viscosities of flexographic inks in use today vary over a wide range, depending on the type of substrate being printed. The relationship between Zahn Cup No. 2 in seconds verses centipoise is illustrated in Chart I.
After determining viscosity, the pressman then manually adds and mixes in solvent with the ink, as necessary, to maintain a constant value of viscosity.
The success of the manual control procedure of adjusting viscosity depends on the skill of the pressman; how often he takes samples and on his ability to judge how much solvent should be added when an increase in viscosity has been observed.
Another problem with flexographic inks is that pigment density in the ink will increase as solvent is lost, with the result that an excess amount of pigment will be used even if film thickness remains constant. Thus, ink mileage also will be adversely affected in those types of flexographic presses in which the metered ink film thickness is not viscosity dependent such as those which use a reverse angle doctor blade and an engraved roller to meter the ink film thickness. This situation also occurs in gravure presses, where the doctor blade wipes the ink from the surface of the printing cylinder leaving ink only in the cells or cavities below the surface of the cylinder.
In short, it is necessary to control the viscosity of flexographic ink during a press run in order to maintain print density and economize on ink usage. There are, however, several additional problems associated with controlling ink viscosity.
First, the system must control viscosity over a relatively wide range - approximately 20-200 centipoise, which corresponds to thin milk on the low end and SAE 20 motor oil at 60.degree. F. on the high end.
Second, the viscosity must be controlled to within .+-.10% of control point in poise e.g., better than .+-.0.5 seconds for a control point of 20 seconds, Zahn Cup #2 and must be designed in such a way that clogging of passages and openings by ink is prevented.
In addition, the device must be easy to clean, have a rugged construction so as to stand up under pressman abuse, be simple to set up for inks of different viscosities and be low in cost.
There are in the prior art numerous devices which have been used to automatically control the viscosity of flexographic and gravure inks. The operating principles of the various types are described below:
One type of prior art device can be referred to as the efflux cup type. This device is an all mechanical controller in which the trajectory of a stream of ink flowing from an orifice under constant head varies with the viscosity. As the viscosity increases, the trajectory shortens, causing the ink to have to flow into an ink cup having a drain hole. When the rate of flow into the cup becomes greater than the drain capability, the cup fills. As the weight in the ink cup increases, the cup drops downward causing the solvent cup to rise, tripping the solvent valve lever, which opens the solvent valve. The flow rate into the solvent cup is greater than its drain capability and the cup fills. In time, the weight of the solvent cup offsets the weight of the ink cup, and the solvent cup drops, automatically shutting off the solvent supply. The ink cup returns to its original up position. The solvent drains from the solvent cup into the ink stream. This general type of device is described and shown in U.S. Pat. No. 2,597,472.
Another type of viscosity control device is the falling piston type. There are several forms of this device. In one form of this device viscosity is measured as a function of the time required for a weighted piston to sink into a sample-filled chamber against the resistance of the fluid as the movement forces the fluid through the annular clearance between the piston and chamber wall. The time for each cycle is a measure of viscosity. The piston is periodically raised by an air-operated mechanism.
The measure of viscosity is electrically transmitted to the controller, where it may be indicated or recorded. When the viscosity exceeds the control point setting, a solvent solenoid valve opens, thereby adding solvent to the ink. The solvent valve contains an adjustable port, which is set to correct the viscosity error without excessive overshooting. The falling piston type is sold by Norcross Corp., Newtown, Mass.
Another type of viscosity control device is based on the time required for longitudinal vibrations of a rod immersed in the ink to subside. A solid-state electronic sub-assembly produces a short pulse of current to a coil situated inside the probe. Once the pulse sets the magnetostrictive blade of the probe in longitudinal motion, no other pulse is generated until the vibration amplitude is dampened to a predetermined level. The "viscosity" of the ink determines when a drive pulse will be generated, thereby providing the base for measurement. That is, the higher the "viscosity" of the ink, the greater the dampening exerted on the vibrating blade, resulting in a more frequent drive pulse being generated. This measurement is reported to the indicator and/or recorder and to the solvent solenoid valve controller in terms of the product of the ink's viscosity and specific gravity. Because viscosity and specific gravity are both inverse functions of temperature, a temperature probe and automatice compensation circuitry are usually employed in conjunction with the viscosity controller. This type of device is sold by the Environmental & Process Instruments Division, Bendix Corp., Lewisburg, W. Virginia under the trade name Ultra-Viscoson Viscometer Model 1800.
A flexural viscosity controller includes a detector consisting of a U-shaped spring rod immersed in the ink stream. One end is excited at 120 hertz by a pulsating magnetic field. The amplitude of the vibration depends on the viscosity of the ink. When the viscosity increases, the resistance to the shearing action of the probe increases and the amplitude of vibration decreases. At the detector, the vibration of the pick-up armature in the field of a permanent magnet induces a 120 hertz voltage in the coil which is proportional to the amplitude of vibration of the probe and thus to the viscosity of the ink. This output signal is converted to a millivolt DC signal to be compatible with standard recorders and controllers. Such a device is sold by Automation Products, Inc., Houston, Texas, under the trade name Dynatrol.
A torsional type of viscosity control device includes variable gain amplifier which drives a magnetic coil that causes a spherical torsional member to oscillate at its natural frequency in the ink pipeline or reservoir. The amplitude of the oscillations is sensed as an AC voltage by an amplitude-monitoring circuit. The voltage is rectified and referenced to a DC voltage equivalent to that obtained when the sphere oscillates in air. The resulting error signal is used to control the gain of the amplifier in order to maintain the amplitude of mechanical oscillation at the reference level over a wide range of viscous loss. The viscosity is obtained from the power required for the magnetic coil to maintain the constant amplitude. U.S. Pat. Nos. 3,382,706; 3,762,429, 3,712,117; and 3,710,614 relate to such torsional type devices.
In another torque type device the viscosity is measured by sensing the viscous drag imposed on a disc or cylinder rotated in the ink at constant speed through a torsion element. An increase in viscosity will cause a beryllium copper spring in the torsion element to wind up, while a decrease will allow the spring to unwind. The drive is either an air motor or an electric synchronous motor. A pickup converts the torque of the element to a pneumatic signal which is transmitted to a controller and an indicator or recorder. The pneumatic controller opens a solvent valve if the sensed viscosity is greater than the preset valve. Solvent will then be added until the set point is attained at which time the controller closes the valve. The measuring range can be changed by unhooking one spindly from the drive extension and hooking on another spindle. No recalibration is required. Such a device is manufactured by Brookfield Engineering Laboratories, Inc., and sold by Viscosel Corp., Stoughton, Massachusetts.
Another prior art type device includes an electronic unit which measures viscosity in conjunction with a sensing motor fitted with a disc immersed in the ink. The current drawn by the sensing motor is proportional to the motor's torque and hence represents the viscosity of the ink. The current is compared with a preset value by the electronic control unit, and the measured deviation initiates the opening of a solenoid valve, thus causing the addition of the required amount of solvent. This type unit is sold by Controle & Automation Div., Chambon, Orleans-la Source, France, under the trade name D.A.S. 2000.
In another prior art device when the drag of the ink on a rotating cylindrical body decreases the number of revolutions under a preset value, a signal is transmitted to a transistorized regulation unit which opens the electromagnetic valve in the solvent line until the preset revolutions is again attained. The above device is sold by Anderson & Vreeland East, Inc., Fairfield, New Jersey.
In a flow matching type device, the ink enters a measuring cell through a calibrated hole and is discharged through a special nozzle. The flow resistance of the outlet nozzle is greater than that of the inlet hole. This condition causes the level of the ink in the measuring cell to rise until the level is sensed by pneumatic detector which opens a valve to admit solvent. When the preset viscosity is attained solvent feed stops. The desired viscosity is set by adjusting the pneumatic detector elevation by means of a threaded rod. A push button is provided for manual solvent addition. There is also an "optical addition indicator". The above device is sold by Selectra S.P.A., 22059 Robbiate (Co.) via Piave 11, Italy.
The prior art devices, described above, all utilize an ink monitoring principle which is based on a physical phenomena that is dependent or varies with ink viscosity. Examples of such phenomena are the drag exerted on a falling or rotating body in the ink, the trajectory of a stream of ink, and the dampening effect exerted by the ink on a vibrating member immersed in it. Although these techniques are sound in theory, their implementation involves either the use of small passages which can easily become clogged with ink or the use of delicate mechanisms which can easily be damaged.
Viscosity and density are different properties in that viscosity is a measurement of resistance to flow whereas density is a measurement of weight per unit volumes i.e. specific gravity. For liquids of different chemical compositions there is no correlation between viscosity and density. Thus, for example, water has a higher density than oil but has a much lower viscosity.
In Chart II there is charted the specific gravity and the viscosity of eleven (11) liquids of different chemical composition ranging from carbon tetrachloride to water. As Chart II illustrates there is no correlation between density and viscosity for the liquids plotted.
Generally speaking, printing ink consists of pigment, resin and solvent wherein the solvent is less dense or lighter than the other constituents. Moreover, solvent being volatile evaporates so that after a period of time the ink becomes more dense.
It has been found, that for any given printing ink used in flexographic and gravure printing there is a correlation between viscosity and density. This fact is illustrated by Charts III and IV.
In Chart III there is charted viscosity (Zahn Cup No. 2) and specific gravity versus the pounds of solvent per one hundred (100) pounds of a given ink. The solid line illustrates that as solvent is added the viscosity decreases; whereas the dotted line illustrates that as solvent is added the density decreases. While these charts specifically relate to white polyamide ink for use on plastic film, it has been found that the same phenomena occurs with respect to other flexographic and gravure inks.
Thus, it has been found that as a solvent such as ethyl alcohol is added to a given printing ink there is a definite relationship between density and viscosity as can be seen in Chart IV which plots density and viscosity of an ink to which ethyl alcohol is added. Thus although there is no correlation between density and viscosity for fluids of different chemical compositions, there is a correlation for a fluid of given composition when that composition is varied by changing the concentration of one of the constituents.
It has been determined through measurements that a ten (10) percent variation in ink viscosity (when measured in centipoise) can be detected by measuring the corresponding variation in ink density. In addition, it has been found that hydrometer structures can regulate ink viscosities and that a hydrometer can be adjusted so as to regulate a wide variety of ink viscosities. The polyamide ink, for example, is used at viscosities as low as 17 seconds, Zahn Cup #2, and a viscosity control of .+-.1/2 second was necessary for proper control. For this case, the variation in specific gravity, corresponding to a 1/2 second variation in viscosity, is found from Chart IV to be 0.012. Thus, the desired accuracy of the specific gravity controller is .+-.0.012, which is well within the accuracy of the hydrometer control valve. It has been found that the accuracy of the hydrometer control valve is .+-.0.0012 when controlling isopropyl alcohol concentrations in fountain solution.